Because no current flows into the op-amp and because all current at
a node must sum to zero, the current entering the input leg must be equal
to that flowing throught the feedback leg from the input to the output
( ).
We also know that the voltage at the inverting input must be zero
(that is the voltage indicated by the ground shown connected to the
non-inverting input).
From our properties of resistors, then, we know that

Consequently,

which means that we're multiplying the magnitude of the input voltage
by the constant .

Notice that the sign of the voltage has changed.
This is a universal trait of the type of op-amp circuits we'll
be using in analog computers.
As we'll see in the example, it doesn't cause any significant problems
in using the computer.